Fix inference when class and instance match protocol

[ilevkivskyi]

Fixes #14688

The bug resulted from (accidentally) inferring against Iterable for both instance and class object. While working on this I noticed there are also couple flaws in direction handling in constrain inference, namely:

• A protocol can never ever be a subtype of class object or a Type[X]
• When matching against callback protocol, subtype check direction must match inference direction

I also (conservatively) fix some unrelated issues uncovered by the fix (to avoid fallout):

• Callable subtyping with trivial suffixes was broken for positional-only args
• Join of Parameters could lead to meaningless results in case of incompatible arg kinds
• Protocol inference was inconsistent with protocol subtyping w.r.t. metaclasses.

[ilevkivskyi]

OK, so the starlette thing used to work because of a chain of truly bizarre "coincidences" (bugs compensating bugs). It will take a while to unwind all this. This PR exposed a whole can of worms.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/arrays/categorical.py:2939: error: Unused "type: ignore" comment  [unused-ignore]

django-stubs (https://github.com/typeddjango/django-stubs): 1.42x faster (55.6s -> 39.3s in a single noisy sample)

steam.py (https://github.com/Gobot1234/steam.py)
- steam/ext/commands/help.py:36: error: Argument 1 to "__init__" of "Command" has incompatible type "Callable[[Context[Any], DefaultNamedArg(str | None, 'content')], Coroutine[Any, Any, None]]"; expected "Callable[[None, Context[Any], VarArg(Any), KwArg(Any)], Coroutine[Any, Any, None]] | Callable[[Context[Any], VarArg(Any), KwArg(Any)], Coroutine[Any, Any, None]]"  [arg-type]
- steam/ext/commands/bot.py:203: error: Signature of "dispatch" incompatible with supertype "Client"  [override]
- steam/ext/commands/bot.py:203: note:      Superclass:
- steam/ext/commands/bot.py:203: note:          def dispatch(self, str, /, *args: Any, **kwargs: Any) -> None
- steam/ext/commands/bot.py:203: note:      Subclass:
- steam/ext/commands/bot.py:203: note:          def dispatch(self, event: str, *args: Any, **kwargs: Any) -> None

core (https://github.com/home-assistant/core)
+ homeassistant/components/sensor/const.py:593: error: Dict entry 39 has incompatible type "SensorDeviceClass": "set[UnitOfSpeed | UnitOfVolumetricFlux]"; expected "SensorDeviceClass": "set[type[StrEnum] | str | None]"  [dict-item]
+ homeassistant/components/number/const.py:497: error: Dict entry 39 has incompatible type "NumberDeviceClass": "set[UnitOfSpeed | UnitOfVolumetricFlux]"; expected "NumberDeviceClass": "set[type[StrEnum] | str | None]"  [dict-item]
+ homeassistant/helpers/entity_registry.py:90: error: Unused "type: ignore" comment  [unused-ignore]

[ilevkivskyi]

OK, mypy_primer looks nice. This is now ready for review.

[JukkaL]

Thanks, nice to see a fix to a highly voted issue! Looks good.

[hauntsaninja]

These changes all make sense, thank you for fixing several chained bugs :-)